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  <div class="question_difficulty">
   难度：Medium
  </div>
  <div>
   <h1 class="question_title">
    264. Ugly Number II
   </h1>
   <p>
    Write a program to find the
    <code>
     n
    </code>
    -th ugly number.
   </p>
   <p>
    Ugly numbers are
    <strong>
     positive numbers
    </strong>
    whose prime factors only include
    <code>
     2, 3, 5
    </code>
    .&nbsp;
   </p>
   <p>
    <strong>
     Example:
    </strong>
   </p>
   <pre>
<strong>Input:</strong> n = 10
<strong>Output:</strong> 12
<strong>Explanation: </strong><code>1, 2, 3, 4, 5, 6, 8, 9, 10, 12</code> is the sequence of the first <code>10</code> ugly numbers.</pre>
   <p>
    <strong>
     Note:
    </strong>
    &nbsp;
   </p>
   <ol>
    <li>
     <code>
      1
     </code>
     is typically treated as an ugly number.
    </li>
    <li>
     <code>
      n
     </code>
     <b>
      does not exceed 1690
     </b>
     .
    </li>
   </ol>
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   <h1 class="question_title">
    264. 丑数 II
   </h1>
   <p>
    编写一个程序，找出第
    <code>
     n
    </code>
    个丑数。
   </p>
   <p>
    丑数就是只包含质因数&nbsp;
    <code>
     2, 3, 5
    </code>
    的
    <strong>
     正整数
    </strong>
    。
   </p>
   <p>
    <strong>
     示例:
    </strong>
   </p>
   <pre><strong>输入:</strong> n = 10
<strong>输出:</strong> 12
<strong>解释: </strong><code>1, 2, 3, 4, 5, 6, 8, 9, 10, 12</code> 是前 10 个丑数。</pre>
   <p>
    <strong>
     说明:&nbsp;
    </strong>
    &nbsp;
   </p>
   <ol>
    <li>
     <code>
      1
     </code>
     &nbsp;是丑数。
    </li>
    <li>
     <code>
      n
     </code>
     &nbsp;
     <strong>
      不超过
     </strong>
     1690。
    </li>
   </ol>
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